CODEWITHSUNDEEP
javacoding-styledictionarynullkey
Aleksandar Savkov
Aleksandar Savkov

Reputation: 2984

Using NULL as a key value in a Map

I'd like to know if using NULL as a key in a Map object is considered good style and if not what are the alternatives?

Upvotes: 8

Views: 3900

Answers (5)

Dave Newton
Dave Newton

Reputation: 160191

Consider using something like a DefaultedMap that returns a known value if the map does not contain a given key.

Depending on your use case, it might be cleaner than using (a) a null key, or (b) a NullObject. If you describe your actual usecase it might be easier to help.

Upvotes: 1

Ryan
Ryan

Reputation: 2091

In general null keys and null values are both poor style.

The Google Guava site has two great pages that explain the how and why of not using Null's.

LivingWithNullHostileCollections - How to cope with Collections that don't allow null

UsingAndAvoidingNullExplained - Guava tools to use and avoid the use of null, explained.

Quote:

But what if?

What if you find yourself wanting to put a null element into one of these null-hostile beasts?

  • If in a Set or as a key in a Map -- don't; it's clearer (less surprising) if you explicitly special-case null during lookup operations
  • If as a value in a Map -- leave out that entry; keep a separate Set of non-null keys (or null keys)
  • If in a List -- if the list is sparse, might you rather use a Map?
  • Consider if there is a natural "null object" that can be used. There isn't always. But sometimes. example: if it's an enum, add a constant to mean whatever you're expecting null to mean here.
  • Just use a different collection implementation, for example Collections.unmodifiableList(Lists.newArrayList()) instead of ImmutableList.
  • Mask the nulls (this needs more detail)
  • Use Optional

Upvotes: 0

Laf
Laf

Reputation: 8185

I would personally recommend using a Null Object (see Null Object Pattern), or some sort of default key, rather than null as a key. Using null always means you have to code to defend against NullPointerException, and should be avoided whenever possible.

Edit

As an example, if you are creating an API that will allow users to add values to a map (be it a home made Map implementation, or a utility class that uses a map to store its data) and your first version let them use null as a key, you will always have to support null keys. So either your backing Map will have to be an implementation that accepts null keys, or you will have to trap suck keys, and replace them with a different key that will represent a null key.

As @Erick Robertson mentionned, there are cases where accepting null keys makes sense. However, if you are designing an API, you should really make sure that you want to handle such keys, as it always means more code to check for null. And it also means more defensive code for the clients of your API.

Upvotes: 3

amukhachov
amukhachov

Reputation: 5900

Sometimes it's useful to store default value in map as a value of Null key. I don't know other cases. But it's legal so why you shouldn't if it can make some profit?

Upvotes: -1

Erick Robertson
Erick Robertson

Reputation: 33068

This is generally not considered good style.

Since NULL typically indicates an undetermined or unset value, it is generally confusing to use as a key to a map. (although, there may be specific circumstances where it makes sense)

The alternatives depend on the situation, so let me use an example. Let's say that we want color specific strings in a text, and we also want a default color for text which doesn't match any of the strings.

private HashMap<String,Color> stringColors;
private Color defaultColor;

Rather than storing the default color in the HashMap using a NULL key, put the default color in the specific variable. This makes it really clear to anyone viewing the code exactly what this color means.

The driving factor, I would say, is if you will actually have a NULL value somewhere that can be directly looked up in the map. (this doesn't happen often, in my experience) In this specific example I gave, the default color would be used for any strings which aren't in the map. In this case, there isn't a NULL string that you want the color for.

Upvotes: 8

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